1. Technical Field
The present disclosure relates to wireless transmissions and more specifically to supplemental signal transmissions within a coverage hole caused by transmission signal interference between a strong local signal and a weak remotely transmitted signal.
2. Introduction
Currently broadcast stations, such as television transmitters, can only share adjacent channel allocations if they are co-located. For example, assume that a first TV station broadcasts a first channel or signal from an antenna located on tower A of FIG. 2 and a second TV station broadcasts a second channel from an antenna on tower A. When two transmitters are co-located in this fashion, the signal strengths of each transmitted signal decline more or less equally with distance away from the transmitter. Receivers such as television antennas are able to distinguish between the two channels when their relative signal strengths at the receiver are equal or approximately equal.
A problem can occur however if two transmission antennas are not co-located. Many markets have the configuration set forth in FIG. 2 in which an antenna on tower A transmits a first channel and an antenna on tower B transmits a second channel. In this case, the signal strength of the signal transmitted from tower A in a near field region around tower A is much greater than the signal strength of the signal transmitted from tower B. FIG. 2 shows a delineated area 208 in which a “coverage hole” occurs. In area 208, a receiver such as receiver 516 at point F has difficulty in receiving the signal from tower B because the signal from tower A is much stronger and interferes with the tower B signal. A similar coverage hole occurs around tower B as shown by area 206. In this area, the signal transmitted from tower A is much weaker and thus difficult for a receiver 518 at position G to receive.
The existence of coverage holes is especially pronounced with adjacent channels in which the frequency of one channel is near the frequency used by the other channel. This interference is shown by way of example in FIG. 3 in which a graph 300 illustrates the frequency/power graph for a signal 302 transmitted from tower A as channel A and an adjacent signal 304 transmitted from tower B on channel B. The interference shown in graph 300 applies to signals in region 206 of FIG. 2. The signal 302 is the weaker signal in the region 206 because its transmission antenna is remote and on tower A. In region 206, channel B (transmitted by tower B) is a stronger signal since its transmission tower B is local. Thus, a receiving device 518 at position G in region 206 would have difficulty receiving the signal 302 broadcast on channel A from remote tower A due to the interference caused by the close proximity and signal strength of signal 304 from tower B. Channel A and B in FIG. 3 are shown as being adjacent to each other which further causes interference between the two channels.
Channels are specific frequency bands, such as a 6 MHz wide allocation between 174 MHz and 180 MHz assigned to channel 7, for example. Transmitters can transmit one or more signals on a particular channel. A receiving station receives and processes the signal to produce an audio program, text, television program, and/or some other form of data. Analog televisions channels are typically 6, 7 or 8 MHz in bandwidth.
One attempt to reduce the interference between channels includes allocating a guard band or channel between the two adjacent channels. Guard bands are used both for terrestrial based communication and satellite communication. Such a guard band would not be needed for adjacent channels if both adjacent channels were transmitted at the same power and height from the same location. However, when adjacent channels are transmitted from different locations, then a guard band is required to enable reception of unrelated channels. FIG. 4 illustrates 400 the use of the guard band in between channels A and B. While this provides some benefit to reducing inter-channel interference, the use of such a guard channel wastes valuable space in the spectrum. For example, if the tower configuration shown in FIG. 2 were deployed to a city, then the available stations for that city may be limited to every other channel. In other words, the Federal Communications Commission (FCC) may allocate channels 2, 4, 6, 8, 10 and so forth. The guard bands allocated by the FCC are represented by channels 3, 5, 7, 9, etc. and prevent the interference between the channels. As can be seen, as more channels are provided in a market, more guard bands and thus more wasted spectrum must be allocated.
What is needed in the art is a new approach that eliminates the coverage holes near transmission towers and frees up additional spectrum because of the allocation of guard channels.